Field of the Invention: The present invention relates generally to drill bits for drilling subterranean formations and, more specifically, to preferentially positioned and oriented elongated bearing elements for rotary drag bits and rotary drag bits so equipped.
State of the Art: Rotary drag bits employing polycrystalline diamond compact (PDC) cutters have been employed for several decades. PDC cutters are typically comprised of a disc-shaped polycrystalline diamond xe2x80x9ctablexe2x80x9d formed on and bonded under high-pressure, high-temperature conditions to a supporting substrate such as cemented tungsten carbide (WC), although other configurations are known. Rotary drag bits carrying PDC cutters, which may be brazed into pockets in the bit face or blades extending from the face or mounted to studs inserted into the bit body, have proven very effective in achieving high rates of penetration (ROP) in drilling subterranean formations exhibiting low to medium compressive strengths. Recent improvements in hydraulic design of rotary drag bits, cutter design and drilling fluid formulation have reduced prior, notable tendencies of such bits to xe2x80x9cballxe2x80x9d by increasing the volume of formation material which may be cut before exceeding the ability of the bit and its associated drilling fluid flow to clear the formation cuttings from the bit face.
Numerous attempts using varying approaches have been made over the years to protect the integrity of diamond cutters and their mounting structures, to limit cutter penetration into a formation being drilled, and to generally stabilize rotary drag bits during operation. For example, from a period even before the advent of commercial use of PDC cutters, U.S. Pat. No. 3,709,308 discloses the use of trailing, round natural diamonds on the bit body to limit the penetration of cubic diamonds employed to cut a formation. U.S. Pat. No. 4,351,401 discloses the use of surface-set natural diamonds at or near the gage of the bit as penetration limiters to control the depth of cut of PDC cutters on the bit face. Other patents disclose the use of a variety of structures immediately trailing PDC cutters (with respect to the direction of bit rotation) to protect the cutters or their mounting structures: U.S. Pat. Nos. 4,889,017, 4,991,670, 5,244,039 and 5,303,785. U.S. Pat. No. 5,314,033 discloses, inter alia, the use of cooperating positive and negative or neutral back rake cutters to limit penetration of the positive rake cutters into the formation. Another approach to limiting cutting element penetration is to employ structures or features on the bit body rotationally preceding (rather than trailing) PDC cutters, as disclosed in U.S. Pat. Nos. 3,153,458, 4,554,986, 5,199,511 and 5,595,252.
In another context, a bearing surface on the bit body is aligned with a resultant radial force generated by cutters thereon to preclude so-called bit xe2x80x9cwhirlxe2x80x9d by maintaining contact of the bearing surface with the wall of the borehole. See also U.S. Pat. Nos. 4,982,802, 5,010,789, 5,042,596, 5,111,892, 5,131,478 and 5,402,856.
So-called xe2x80x9cwear knotsxe2x80x9d have been deployed behind PDC cutters on the face of a rotary drag bit in an attempt to provide enhanced stability in some formations, notably interbedded soft, medium and hard rock. Drill bits drilling such formations easily become laterally unstable due to the wide and constant variation of resultant forces acting on the drill bit due to engagement of such formations with the PDC cutters carried on the face. Wear knots comprise structures in the form of bearing elements projecting from the bit face and which conventionally rotationally trail some of the PDC cutters at substantially the same radial locations, usually at positions from the nose of the bit extending down the shoulder to proximate the gage. Conventionally, wear knots may comprise elongated segments having arcuate, such as half-hemispherical, leading ends, taken in the direction of bit rotation. The wear knots project from the bit face a lesser distance than the projections, or exposures, of their associated PDC cutting elements and are typically of lesser width than a rotationally leading, associated PDC cutter and thus, consequently, than the groove in the formation cut by that PDC cutter. One notable deviation from such design approach is disclosed in U.S. Pat. No. 5,090,492, wherein so-called xe2x80x9cstabilizing projectionsxe2x80x9d rotationally trail certain PDC cutters on the bit face and are sized in relation to their associated cutters to purportedly snugly enter and move along the groove cut by the associated leading cutter in frictional, but purportedly noncutting relationship to the side walls of the groove.
It has been observed by the inventor herein that the presence of bearing elements in the form of wear knots, while well-intentioned in terms of enhancing rotary drag bit stability, falls short in practice due to deficiencies in the positioning and orientation of the wear knots. Notably, current wear knot designs and placements, rather than the wear knot riding completely within a groove cut by an associated, rotationally leading PDC cutter, result in some portion, and in some instances a substantial portion, of each wear knot exterior surface contacting the uncut rock adjacent the groove and exciting, rather than reducing, lateral vibration of the bit.
The present invention provides a well-reasoned, easily implementable bit design particularly suitable for rotary drag bits, which bit design may be tailored to a specific ROP or range of ROPs. In the inventive bit design, lateral positioning and angular positioning of elongated bearing elements on the bit face as well as their respective dimensions and configurations may be adjusted so that all portions of an elongated bearing element travel completely within a tubular clearance volume defined by the path through the formation being drilled by a PDC cutter with which that elongated bearing element is associated, the PDC cutter being positioned at about the same radius from the bit centerline as the elongated bearing element.
PDC cutters of a rotary drag bit travel along helical paths as the bit drills ahead into the formation. The helix pitch of a cutter, in units of distance (for example, inches) per revolution is related to ROP and bit rotational speed, measured in revolutions per minute. The determined pitch, in combination with a radial position of a PDC cutter on the bit, may be used to determine the slope of the helical path cut by that cutter in the formation.
The elongated bearing elements of the present invention may rotationally trail an associated PDC cutter in an immediately circumferentially adjacent position, or by a substantial arc around the bit face. In practice, an elongated bearing element may trail an associated PDC cutter by a circumferential distance in excess of 180xc2x0, so as to actually be rotationally leading another, different PDC cutter traveling along a path lying at about the same radius from the centerline of the bit as the elongated bearing element.
In one embodiment, elongated bearing elements may be configured as elongated, half-cylindrical segments having non-aggressive leading and trailing ends to preclude inadvertent cutting of formation material, for example, a rotationally leading half-hemispherical end and a rotationally trailing half-hemispherical end. Each elongated bearing element substantially correspond to a portion of a circular path traversed by an associated PDC cutter at substantially the same radius as the bit rotates. The outermost face, or bearing surface, of each elongated bearing element may be oriented (as the bit is normally situated during drilling) at an angle with respect to the centerline generally corresponding to the slope of the helical path traversed by its associated PDC cutter for a given ROP or designed range of ROPs as the bit drills ahead into the formation.
It is specifically contemplated that elongated bearing elements according to the pesent invention may be applied to coring bits, bi-center bits, eccentric bits, reaming tools and other drilling structures as well as to full-bore drill bits. As used herein, the term xe2x80x9cbitxe2x80x9d encompasses all of the foregoing drilling structures. Moreover, the present invention is not limited to any particular structure for rotary drag bits and may be applied with equal utility both to matrix-type rotary drag bits as well as steel body bits and bits of various constructions, including without limitation bits formed by stereolithographic, or so-called xe2x80x9clayered manufacturing,xe2x80x9d techniques, as known in the art.
Methods of designing rotary drag bits are also encompassed by the present invention, as are methods of drilling.